Method of operating an electric arc furnace



MaICh 1968 w. H. BOECKLER ET AL. 3,373,240

METHOD OF OPERATING AN ELECTRIC ARC FURNACE Filed June 1, 1964 V L 11' lD I I INVENTORS. Wa/rer H. Boe'ck/el" G/Iber/ .5. L ayn a United StatesPatent ABSTRACT OF THE DISCLOSURE An improved method of operating anelectric arc furnace by immersing a hollow electrode below the surfaceof the melt and passing an inert gas through the hollow electrode toform an envelope of inert gas between the electrode and the melt. Byvarying the flow rate of the inert gas and consequently varying the sizeof the gas envelope, the electrical resistance between the electrode andthe bath is controlled thereby controlling the power to the electric arcfurnace.

This invention relates to an improved method of operating an electricarc furnace of the type wherein an arc is drawn between the charge orbath in the furnace and at least one electrode, and particularly to sucha method employing an inert gas to provide a high resistance envelopearound at least one electrode of said are furnace.

A preferred embodiment of the present invention is schematically shownin the drawing which illustrates an elevation, partially in section, ofan electric arc furnace.

Operation of electric arc furnaces generally may be separated into twodistinct phases. The first or start up phase, generally consists ofpassing electric current through the charge in the furnace, thereby toheat the charge by means of electrical resistance. No electric arc isusually employed during this phase of operation. After a portion of thecharge in the furnace has been heated sufiiciently to form a moltenbath, the second phase of operation begins. In the second phase, anelectric arc is drawn between an electrode and the molten bath. It is tothis second phase that the present invention is directed.

An electric arc furnace of the type herein contemplated may be thoughtof as .a series circuit containing a plurality of resistances. Power,then is the product of the square of the current (1 and the overallresistance of the circuit (R). Overall resistance under ordinaryoperation is the sum of lead resistance (R and bath resistance (R Whenelectrode and hearth resistances are neglected. Thus, Power=I (R +R Whenan electric arc furnace is operated in the conventional manner, an arcis drawn between the electrode and the molten bath. As the bath heatsup, its electrical resistance R becomes lower. In order to maintain aconstant power input to the furnace, then, the electrical current mustbe increased. Increase of current input to the furnace causes heating ofthe leads and increases their electrical resistance R Power is then lostto the leads and more must be supplied in order to continue heating thebath. As more power is supplied to the system and the bath continues toheat up, the cycle repeats itself, thereby requiring increasing amountsof electrical current to maintain heating operation of the arc furnaceand increasing the possibility of damage to the electrical leads.

It is .an object of the present invention to provide a method ofoperating an electric arc furnace whereby power input to the furnace maybe maintained substantially constant without substantial increase incurrent input.

Other objects and advantages of the present invention will becomeapparent in the course of the following specification.

The Figure of the drawing presents an embodiment of an arc furnace forcarrying out the present novel process. In the embodiment illustrated, amolten bath 10 is positioned in a suitable crucible or hearth 11 made ofan electrically conducting material. An electrode 12 is positioned sothat at least a portion thereof is within the crucible or hearth 11 andimmersed in the molten bath 10. Lead wires 14 provide externalcommunication between hearth 11 and electrode 12 through a source ofelectrical power. The electrode 12 is further characterized by a bore 15connected to a source of inert gas.

In accordance with the present invention, an inert gas is passed throughthe bore 15 of the electrode 12 during second phase operation of thefurnace. An envelope 16 of inert gas is thus formed around the exteriorof the electrode 12, especially the portion of electrode 12 in closeproximity to the molten bath 10. By this means, an additional resistance(R is placed in series with the furnace electrical circuit, whichresistance may be controlled so as to maintain current and powersubstantially constant. An increased flow of inert gas through the bore15 of electrode 12 increases resistance R and, conversely, a decrease inthe flow of inert gas decreases resistance R As the bath resistance Rdecreases with increasing bath temperature, R may be increased to makeup for the loss in bath resistance. In this manner, the power input [I(R +R +RG)] may be held substantially constant without increasing thecurrent flow in the system or altering the position of the electrode 12relative to the bath 10.

Suitable inert gases for use in the present invention are wholly orpartially ionizable gases which do not react with the electrode 12, thehearth 11, or the bath 10 to any significant extent at the temperaturesemployed. Preferred inert gases for use in the present method arehelium, neon, .argon, krypton, Xenon, radon, nitrogen and mixturesthereof.

A better understanding of the present method may be obtained in light ofthe following examples which are set forth to illustrate, and are not tobe construed to limit, the present invention.

Example 1 A four-inch diameter impervious graphite hearth was chargedwith 1.5 kilograms of bauxite, pyrites and coke. The desired reactiontemperature of these materials is about 1850 degrees centigrade.Electrical resistance of the furnace circuit, including the charge(bath) resistance (R at start up was about 0.065 ohm.

A cylindrical graphite electrode having a diameter of one inch and a 3&inch diameter axial bore was positioned substantially vertically withits lower end immersed in the charge (bath). Electrical connection wasmade between the graphite hearth and the electrode through an electricalpower source. A source of argon gas was connected to the axial bore ofthe electrode. The electric power was turned on to begin the heatingprocess.

After heating for about 1 /2 hours employing no gas, the bath was moltenand had attained a temperature of about 1850 degrees centigrade.Electrical resistance of the circuit had decreased to about 0.035 ohm.Argon gas was then fed through the bore of the electrode under about 5pounds per square inch pressure. Electrical resistance of the furnacecircuit was increased to about 0.3 ohm thereby, with a correspondentreduction in the amperage required to maintain the bath at the desired(1850 C.) temperature.

It was also found that electrode wear, as compared Example 2 The methoddescribed in Example 1 may be carried out in substantially the samemanner substituting helium, neon, krypton, xenon, radon, nitrogen, ormixtures thereof for the argon employed therein with substantially thesame beneficial results.

Various modifications may be made in the present invention withoutdeparting from the spirit or scope thereof, and it is to be understoodthat We limit our-selves only as defined in the appended claims.

We claim:

1. In a method of operating an electric arc furnace having a hearth andan electrode, said hearth and electrode communicating electrically withan external electric power source through an electrically conductivecharge, the improvement comprising providing said electrade with aninternal longitudinal bore, immersing said i electrode below the surfaceof the charge, and passing an inert gas through said bore duringoperation of said electric arc furnace to produce an envelope of inertgas between the electrode and the charge, thereby to increase the arcresistance and to thereby control the electrical power input to said arcfurnace.

2. The method of claim 1 wherein the inert gas is a member selected fromthe group consisting of helium, neon, argon, krypton, Xenon, radon,nitrogen and mixtures thereof.

References (fitted UNITED STATES PATENTS 2,303,973 12/1942 Armstrong139XR 3,101,385 8/1963 Robinson 13-34 XR FOREIGN PATENTS 221,949 6/1959Australia.

BERNARD A. GILHEANY, Primary Examiner.

R. N. ENVALL, 1a., Assistant Examiner.

